Typically, printed circuit boards that require protection from moisture, electric leakage, dust, and salt fog are coated with insulating films, known as conformal coatings. These conformal coatings are usually made up of such things as acrylic, polyurethane, silicone, or epoxy synthetic resins dissolved in a volatile solvent. When applied to clean circuit boards, an insulated resin film of uniform thickness and without pinholes, is formed as the solvent evaporates.
Numerous methods currently exist for applying coatings of the moisture-proof insulating material to printed circuit boards. A first method is immersing the printed circuit board in an immersion tank containing the insulating material. In a second method, the insulating material is applied to the electronic assembly by a brush. In a third method, a sheep's wool roll impregnated with insulating material is rolled on to the surface of the electronic assembly. In a fourth method, an assembly with pressurized nozzles applies the insulating material to the circuit board. Finally, in a dye method, the insulating material is pressurized and extruded from a slit dye to eject a film for coating the printed circuit board surface.
Spraying is the most commonly used insulating coating method employed in mass production. Spraying can be categorized as either air spraying in which an air stream is impinged against a stream of liquid coating material after leaving the spray nozzle to atomize it, and airless spraying in which the coating material is dispensed in a low pressure, unatomized film pattern.
One traditional approach for application of solvent based conformal coating materials to electrical assemblies for environmental protection has utilized conventional high pressure commercial or industrial spray equipment. In one configuration, an automated spray booth operating six industrial spray guns is used to apply conformal coating to the surface of the assembly as it passes through on a conveyor. In this process, a mixture of conformal coat and thinner/carrier is fed by a positive displacement pump to the spray heads which atomize the solution and apply it directly to the assembly. Due to the design of the spray heads used in this equipment, low volume with high atomization pressure, a low viscosity fluid, i.e., low solids and high solvent content, must be used in order to atomize the solution as it exits the fluid nozzle. The atomization pressure and therefore, high spray velocity, along with the high solvent concentration causes a great deal of overspray and a high transfer loss effect due to particles rebounding from the surface to be coated. A direct result of this phenomenon are low solid transfers to the surface, chemical waste, and high emissions.
All companies which apply conformal coatings in the United States must comply with Environmental Protection Agency (EPA) requirements. These requirements set a limit on the hazardous air pollutants that the company emits during a year. Local air quality regulations set a limit on the maximum amount of volatile organic compound emissions from the plant, as well as limit the hydrocarbon ambient air quality on the property line of a facility. Compliance with these regulations can be quite expensive and time consuming. One method of complying with these regulations is to simply employ the spraying operation less often. This can be a major disadvantage when trying to meet a delivery schedule and there is not enough time to get the product out. Another alternative is to install an exhaust scrubber system. This can prove to have a very large up front cost and require significant yearly maintenance. A final solution is to change the type of volatile organic compounds used. This also has a very high implementation cost and adds significantly to the yearly material cost, because the different characteristics of the new materials require extensive study to prove that they are adequate.
Therefore, the object of the present invention is to develop a method and apparatus which allows for the application of conformal coat to electronic assemblies while reducing the amount of materials wasted and complying with the environmental regulations.